Electron discharge tube circuits for generating electrical oscillations of saw-tooth wave form



Dec. 9, 1952 HUNTLEY 2,621,237 ELECTRON DISCHARGE TUBE CIRCUITS FORGENERATING ELECTRICAL OSCILLATIONS OF SAW TOOTH WAVE FORM Filed Aug. 24,1950 FIG.

lnvehfa, KEITH GORDON HUNTLEY Af/v iney Patented Dec. 9, 1952 FHQEELECTRON DISCHARGE TUBE CIRCUITS FOR GENERATING ELECTRICAL OSCILLA'KIONSOF SAW-TOOTH WAVE FORM Keith Gordon Huntley, Harlington, Hayes, England,assignor to Electric & Musical Industries Limited, Hayes, Middlesex,England Application August 24, 1950, Serial No. 181,186 In Great BritainSeptember 22, 1949 8 Claims. 1

This invention relates to electron discharge tube circuits forgenerating oscillations of sawtooth waveform. Such circuits are used forexample in television for the purpose of reflecting the beam of acathode ray tube.

In a known circuit two tubes are employed for generating the long flankof the sawtooth oscillations, one of the tubes being conducting for oneportion of said long flank and the other tube being conducting for theother, portion of the long flank. A type of circuit which employs twotubes in this manner is one which operates on the resonant returnprinciple in which a sawtooth current is set up in inductive scanningcoils associated with a cathode ray tube and in which the short flank ofthe sawtooth oscillations is obtained by allowing the current in thecoils to execute a free half-cycle of oscillation at the end of the longflank of the sawtooth oscillation. In such a circuit one of theaforesaid tubes is employed to prevent further free oscillation bybecoming conducting, this tube during its conducting period generatingthe initial portion of the long flank of the sawtooth oscillation afterwhich the other tube becomes conducting for generating the remainingportion of the long flank. In this resonant return type of circuit, thetube which prevents further oscillation is usually called the dampertube and the other tube is usually called the driver tube since thislatter tube has applied thereto suitable controlling voltages whichserve to render the valve conducting in an appropriate manner for thegeneration of the sawtooth oscillations. In some circuits of this typewhen employed for generating oscillations at line frequency in atelevision receiver the driver tube may be non-conducting for as much asthe first 50 per cent of the long flank of the generated oscillation. Itis of course well recognised that it is desirable for the long flank ofthe generated oscillations to be linear throughout its whole stroke butit is found with the circuits described above that the initial portionof the long flank is somewhat curved.

In the United States co-pending Application Serial No. 128,598, filedNovember 21, 1949 by J. E. Best and R. C. White, circuits are disclosedfor making the initial portion of the long flank more linear. Thepresent invention seeks to provide improvements in said circuits.

In order that the invention may be clearly understood and readilycarried into eiTect, it will now be more fully described with referenceto the accompanying drawing, in which Figure 1 illustrates an embodimentof the present invention and Figure 2 illustrates a part of the circuitshown in Figure 1 embodying a modification.

The drawing illustrates a circuit according to the invention whichoperates on the resonant return principle and mainly diifers from thecircuits shown in Figures 1 and 2 of the drawings "of said co-pending'application in that the negagenerating controlling voltages which inthis case,

in the absence of negative feedback, are substantially constant voltagesbut which are modified by the negative feedback voltage to set upsubstantially sawtooth voltages for application to the driver and dampertubes.

As shown in Figure l in the drawing the reference numeral l3 illustratesthe driver tube and the reference numeral It the damper tube. The drivertube I3 is a tetrode and its anode is connected to the positive terminalof a source of anode current indicated at l5 through a resistance l5,which serves to suppress parasitic oscil lations, the primary Winding I!of a transformer and a decoupling resistance i8 which is decoupled toearth by a condenser I9. The secondary winding 20 of the transformerfeeds current to the scanning coils 2! through a resistance 22, theresistance of the scanning coils being indicated by the resistance 23.The damper tube I4 is shunted across the scanning coils 2!, and is shownas a triode. Control voltages are applied to the control electrodes ofthe tubes I3 and I4, said control voltages being derived from a doubletriode tube 25!. The tubes l3 and M have their cathodes connected toearth'through resistances 25 and 26 respectively which serve to make thecharacteristics of said tubes more linear. The common cathode of tube 25is connected through a resistance 21 to a source of negative potential,the resistance 21 serving to cause the tube 2a to function as apush-pull cathode coupled amplifier. Assuming that the scanning coils 2l were ideal and that the tubes l3 and It were also ideal i. e. havezero internal impedance, then the voltage across said coils, in order toset up a truly linear sawtooth current in the coils, would be a constantvoltage so that the control voltages derived from the tube 24 and whichare fed. to the control electrodes of the tubes l3 and It would, in thiscase, be constant and no negative feedback voltage would be required.However,

"in thescanning coils 2|. components are'designed so thatC2sR31=L21/R23.

the scanning coils do not represent an ideal inductance and the tubespossess internal impedance and, accordingly, the control voltagesapplied to the tubes 13 and M are modified by a negative feedbackvoltage,.such negative feedback voltage being substantially proportionalto the voltage set up across said scanning coils. The negative feedbackvoltage is derived, as shown, from the upper end of the scanning coils2|, and is applied through condenser 28 and through a resistance 29 toone of the control electrodes of the tube 24. The other controlelectrode'of the tube 24 is connected to earth through a resistance 3!The junction between the. condenserztaand resistance 29 is connected toa resistance 3| :to earth through a decoupling condenser 32, thejunction point of the resistance 3! and condenser 32 being connected toa tapping point on a potentiometer 33 which is connected across a sourceof D. C. potential. The condenser 28 andresistance 31 form a timeconstant circuit in the-feedback path feeding the voltage derived fromthe upper end of the scanning coils 2| to thetube'illl so as'to impartacorrect shape'to this feedback voltage to compensate for theresistance: 23'. of: the scanning coils "and thus obtain the: requiredlinearity of sawtooth current set up 'For this purpose the Thetappingjpoint on the potentiometer '33 is adjustable for the;purposeofvarying the slope .of ;.the. sawtooth current. The: tube .24 functionszasapushrpullamplifier and the control yoltages which zarezzsetzup.:in-Lthe anode circuits of .the

"tube 1.2 4-.':are..applied .as' aforesaid" to 'the -control.elec'itrodes;Qf." the tubes i3and H3.

1'34. of'the tube 2c is connected to thesource'of 1 positive potential:l through resistances 35' and The anode 36, the junction point of theresistances '35 and -3$:. being decoupled to earth througha condenser -3'l. electrode'of the tube 13 through a rectifier 38 -whichimay bea-contact rectifier or a rectifier The anode 35 is connected to thecontrol ofsome ,other'suitable form and a'resistance 39 "which. isshuntedby a bypass condenser 40, the

controkelectrode circuit of the tube 13 including I airesistance 4 l.and a resistance 42 to the junction of-"whichi the resistance as isconnected, whilst thelowerqend of the resistance-42 is connected toratapping point on a resistance 43 one end of.which-31s; connected to thenegative terminal :of: a'sou-rce of: bias voltage whilst the other endsource of anode potential through resistances "M5 and- 41 thejunction'point of which-is decoupled-to earth through a condenser 43.The

l anodeAE is connected to. the control electrode of theatube: l4 througha resistance I353 shunted by a bypass condenser 59,- the resistance libeing connected, to the junction of 1 a. resistance 5 Land -aresistance:52,.the other end ofv the latter being :connected to a sourceof negative potential, the

resistancesllfi and 52 serving to apply an appropriatev biassingpotential to the tube M. The potentialfor thescreening electrode ofthetube l3 is .derived from the positive potential source I5 through aresistance 53, the screening electrode .beingdecoupledto thecathodeofthe tube l3 through condenser 56. Thecircuit shown is syn- Between the'recchronised by applying to the control electrode of the tube I3synchronising pulses which are set up at the anode of a tube 55 whichmay be the output tube of a multivibrator or other source of pulses, theanode of thetube 55 deriving its anode potential from a resistance 56connected tothe junction point of the resistances and 3E.

The condenser 44 serves to prevent undesired oscillations which may beotherwise set up due to phase shift in the feedback network and the rec-'tifiertuserves'to isolate the condenser 64 from the tube when a pulseis set up at the anode 'ofsaid tubeso as to prevent delay in the buildapulse is applied to the control'electrode of the .tube i3 from the tubethe tube [3 is rendered non conducting and the current in the scanningcoils 2i is then permitted. toexecutea half-cycle ofxosei'llation afterwhich the tube l4 becomes conducting to permitthe'generation of theinitial portion of the-long flank of the sawtooth current across thecoils 2i, this'portion'of the long. flank being controlled andlinearised by the negative feedback voltage fed to the tube-'24. Thisfeedback voltage isuof course amplified by :thatube .24; and? serves toreduce thezeilective; impedance ..of. thetube l-il. so-..-that atsubstantiallyilinear. curzrentilcwsthroughthe-coils 2 lThe.-.-D.71C.;bias applied-to; the tube; 5 3 is so adjusted. that beforethe tube-4 l is rendered'nonsconducting' the tube V i3-becon1esconducting so as to generate the subsequent portio-n ofthe long flank ofthelsawtoothcurrent. This subsequent portion isualso lineariseddue tothe-negative feedback which is amplified by thetubei l and fed from theanode 33"to the control'electrode-of tube 13. Upon the occurrence of afurther pulse from the tube 55,

theltube 131-is' rendered non-conducting rand the :cycle recommences.

The resistance. 46 is made considerably smaller .thanthe. resistance-35so that agreater' degree vent undesired oscillations from occurring.

Theci-rcuitshown in Figure 1 may be modified as shown in Figure 2- byproviding a resistance ii? in the anode :circuit of: the tube l4 andthen connecting .the upper-end of the scanning coils 2i to'the. junctionof the resistance-'57- andthe anode ofthe tubeal. The resistance:5li.is."made equal to i theeffective 'xanoderesistance .of the tubeli,.i. e., the actualanode resistance divided by the :gain of thefeedback, loopz'so :that .the upper end of the coils2l...is.:theneifeotively a point-of zero: impedance thus :Zfurther:improving .the. linearity of the *generated' sawtooth oscillations.

'W-hatI claim is: l. A circuit for generating oscillations of sawtoothwaveform .in a load, comprising a pair of tubes each having acontrol-electrode, a source of control voltage, means. for applying acontrol voltage'frorn said sourceto one of said tubes to control thegeneration of the initial portion of the long flank of .a-sawtoothoscillationymeans for applying a control voltage from said source to theother tube to control the subsequent portion of said long flank, aconnection from said load to provide a negative feedback voltagesubstantially proportional to the Voltage set up across said load, saidsource of control voltage including a constant voltage, and means forobtaining the difference between said negative feedback voltage and saidconstant voltage to derive said control voltage modified to linearizeboth the initial and subsequent portions of said long flank.

2. A circuit for generating oscillations of sawtooth waveform in aninductive load, comprising a driver tube having a control electrode,means for rendering said tube conducting to set up a portion of the longflank of a sawtooth oscillation in said load, means for interrupting thecurrent in said load to produce a free half cycle of oscillation of saidcurrent for generating the short flank of said sawtooth oscillation, adamper tube connected across said load to prevent further freeoscillation and to generate a further portion of said long flank, saiddamper tube having a control electrode, a source of control voltage,means to provide a negative feedback voltage substantially proportionalto the voltage set up across said load, said source of control voltageincluding a constant voltage, means for obtaining the difference betweensaid negative feedback voltage and said constant voltage to derive saidcontrol voltage, and means for applying said control voltage to thecontrol electrodes of said driver and damper tubes to linearize bothsaid portions of said long flank.

3. A circuit for generating oscillations of sawtooth waveform in a load,comprising a pair of tubes each having a control electrode, a source ofcontrol voltage, means for applying a control voltage from said sourceto one of said tubes to control the generation of the initial portion ofthe long flank of a sawtooth oscillation, means for applying a controlvoltage from said source to the other tube to control the subsequentportion of said long flank, said source including a push-pull amplifierfor generating said control voltage, a connection from said load to saidamplifier to provide a negative'feedback voltage substantiallyproportional to the voltage set up across said load, means for applyingsaid feedback voltage to said amplifier to obtain a control voltagedependent on the difference between said negative feedback voltage and aconstant voltage, and means for applying said control voltage inpush-pull to said tubes to linearize both the initial and subsequentportions of said long flank.

4. A circuit for generating oscillations of sawtooth waveform in aninductive load, comprising a driver tube having a, control electrode,means for rendering said tube conducting to set up a portion of the longflank of a sawtooth oscillation in said load, means for interrupting thecurrent in said load to produce a free half cycle of oscillation of saidcurrent for generating the short flank of said sawtooth oscillation, adamper tube connected across said load to prevent further freeoscillation and to generate a further portion of said long flank, saiddamper tube having a control electrode, a source of control voltageincluding push-pull amplifier, means coupling said load to said sourceto provide a negative feedback voltage substantially proportional to thevoltage set up across said load, means for applying said feedbackvoltage to said amplifier to obtain control voltage dependent on thedifference between said negative feedback voltage and a constantvoltage, and means for applying said control voltage in push-pull tosaid tubes to linearize both said portions of said long flank.

5. A circuit for generating oscillations of sawtooth waveform in aninductive load which includes a resistive component, comprising a drivertube having a control electrode, means for rendering said tubeconducting to set up a portion of the long flank of a sawtoothoscillation in said load, means for interrupting the current in saidload to produce a, free half cycle of oscillation of said current forgenerating the short flank of said sawtooth oscillation, a damper tubeconnected across said load to prevent further free oscillation and togenerate a further portion of said lonk flank, said damper tube having acontrol electrode, a source of control voltage, means coupling said loadto said source to provide negative feedback voltage substantiallyproportional to the voltage set up across said load, said source ofcontrol voltage including a constant voltage, means for obtaining thediflerence between said negative feedback voltage and said constantvoltage to derive said control voltage, and means for applying saidcontrol voltage to the control electrodes of said driver and dampertubes to linearize both said portions of said long flank, said couplingmeans including a time constant network to compensate for the resistivecomponent of said load.

6. A circuit for generating oscillations of sawtooth Waveform in aninductive load which includes a resistive component, comprising a drivertube having a control electrode, means for rendering said tubeconductive to set up a portion of the long flank of a sawtoothoscillation in said load, means for interrupting the current in saidload to produce a free half cycle of oscillation of said current forgenerating the short flank of said sawtooth oscillation, a damper tubeconnected across said load to prevent further free oscillation and togenerate a further portion of said long flank, said damper tube having acontrol electrode, a source of control voltage including a push-pullamplifier, means coupling said load to said source to provide a negativefeedback voltage substantially proportional to the voltage set up acrosssaid load, means for applying said feedback voltage to said amplifier toobtain a control voltage dependent on the difference between saidnegative feedback voltage and a constant voltage, and means for applyingsaid control voltage in push-pull to said tubes to linearize both theinitial and subsequent portions of said long flank, said coupling meansincluding a time constant network to compensate for the resistivecomponent of said load.

7. A circuit for generating oscillations of sawtooth waveform in aninductive load comprising a driver tube having a control electrode,means for rendering said tube conducting to set up a portion of the longflank of a sawtooth oscillation in said load, means for interrupting thecurrent in said load to produce a free half cycle of oscillation of saidcurrent for generating the short flank of said sawtooth oscillation, adamper tube connected across said load to prevent further freeoscillation and to generate a further portion of said long flank, saiddamper tube having a control electrode, a source of control voltage,means coupling said load to said source to provide negative feedbackvoltage substantially proportional to the voltage set up across saidload, said source of control voltage including a constant voltage, meansfor obtaining the difference between said negative feedback voltageandusaid constant voltage toderive saidcontrol voltage, and means forapplying said control voltage to the control electrodes .ofsaid driverand damper tubes to linearize both portions of said long flank, saiddamper tube having ananode coupled to said load through a resistance,and one end. of said load being connected to the junction of saidresistance and anode to maintain said end effectively at zero impedance.

8. A circuit for generating oscillations of savvtcoth waveformin aninductive load comprising a driver tube having a control electrode,means for renderingsaidtube conducting to set up a portion of the longflank of a sawtooth oscillation in said load, means for interrupting thecurrent in said load to produce a free half cycle of oscillation of thecurrent for generating the short flank of said sawtooth oscillation, adamper tube connected across said load to prevent further freeoscillation and to generate a further portion of said long flank, saiddamper tube having a control electrode, a source of control voltageincluding a push-pull amplifier, means coupling said load to said sourceto provide a negative feedback voltage substantially proportional to thevoltage set up acrosssaid load, said source of control voltage includinga constant voltage, means for obtaining the difierence between saidnegative feedback voltage and said constant voltage to derive saidcontrol voltage, and means for applying said control voltage inpush-pull to said tubes'to linearize both said portions of .said longflank, said damper tube having an anodecoupled to said load through aresistance; and one end of said load being connected to the junction ofsaid resistance and anode to maintain said end effectively at zeroimpedance.

KEITH GORDON HUNTLEY.

REFERENCES CITED The following references are of record in the file ofthis patent:

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